Diseases of plants have caused an ongoing and constant problem in plant cultivation. The fungal pathogen, Sclerotinia sclerotiorum, in particular is said to cause disease in nearly 400 plant species. Sclerotinia sclerotiorum appears to be among the most nonspecific, omnivorous, and successful of plant pathogens. (Purdy, L. H., Phytopathology 69: 875-880 (1979))
Sclerotinia infections in sunflower, for example, are considered the major disease problems of the crop yet little genetic resistance is currently available to breeding programs to combat the various forms of this fungal infection. In fact, there are no major gene resistance mechanisms that have been defined in any species affected by this pathogen.
Oxalate (oxalic acid) is a diffusable toxin associated with various plant diseases, particularly those caused by fungi. While some leafy green vegetables, including spinach and rhubarb, produce oxalate as a nutritional stress factor, certain pathogens synthesize and export large amounts of oxalate to assist in the establishment and spread of the organism throughout infected hosts. Oxalate is used by pathogens to gain access into and subsequently throughout an infected plant. See for example, Mehta and Datta, J. Biol. Chem., 266: 23548-23553, and published PCT application WO 92/14824 published in Sep. 3, 1992. Field crops such as sunflower, bean, canola, alfalfa, soybean, flax, safflower, peanut, clover, maize, sorghum, wheat, rice, as well as numerous vegetable crops, flowers, and trees are susceptible to oxalate-secreting pathogens. For example, fungal species including, but not limited to, Sclerotinia, Sclerotium, Aspergillus, Streptomyces, Penicillium, Pythium, Pacillus, Mycena, Leucostoma, Rhizoctonia and Schizophyllum use oxalic acid to provide an opportunistic route of entry into plants, causing serious damage to crops such as sunflower.
Enzymes that utilize oxalate as a substrate have been identified. These include oxalate oxidase (wheat oxalate oxidase is sometimes called germin) and oxalate decarboxylase. Oxalate oxidase catalyzes the conversion of oxalate to carbon dioxide and hydrogen peroxide. A gene encoding barley oxalate oxidase has been cloned from a barley root cDNA library and sequenced (See: PCT publication No. WO 92/14824, published in Sep. 3, 1992). A gene encoding wheat oxalate oxidase activity has been isolated and sequenced, and the gene has been introduced into a canola variety (PCT publication No. WO 92/15685 published in Sep. 17, 1992, Drawtewka-Kos, et al., J. Biol. Chem., 264(9): 4896-4900 (1991)). Oxalate decarboxylase converts oxalate to carbon dioxide and formic acid. A gene encoding oxalate decarboxylase has been isolated from Collybia velutipes (now termed Flammulina velutipes) and the cDNA clone has been sequenced (WO 94/12622, published in Jun. 9, 1994). In addition, another oxalate decarboxylase gene has been isolated from Aspergillus phoenices (U.S. patent application Ser. No. 08/821,827, filed on Mar. 21, 1997).
Another gene which does not degrade oxalate, but which has been shown to help in the control of plant fungal pathogens is glucose oxidase. (See U.S. Pat. No. 5,516,671, filed on Nov. 3, 1994 and Wu, et al., Plant Cell, 7: 1357-1368 (1995)). In the presence of oxygen, glucose oxidase catalyzes the oxidation of glucose to .sigma.-gluconolactone and hydrogen peroxide. It is thought that the hydrogen peroxide and the .sigma.-gluconolactone, which is known as glycosyltransferase inhibitor, are responsible for the anti-pathogenic mode of action.
In many plants, attempted infection by avirulent pathogens triggers the activation of multiple defenses that may be accompanied by a hypersensitive response (HR) or collapse of host tissue around the site of pathogen penetration. A consequence of these responses is a restriction of pathogen spread within the host and frequently development of systemic acquired resistance (SAR) to subsequent infection by pathogens that may be taxonomically distant to the initial pathogen. For e.g., SAR induced by virus inoculation may be effective against subsequent attack by bacterial or fungal pathogens or vice versa. One of the earliest responses of the plant to infection is an oxidative burst which can be detected as an increased accumulation of superoxide (O.sub.2) and/or hydrogen peroxide (H.sub.2 O.sub.2). O.sub.2 is very reactive and can form other reactive oxygen species, including hydroxyl radical (OH) and the more stable H.sub.2 O.sub.2. H.sub.2 O.sub.2 accumulation may trigger enhanced resistance responses in a number or ways: 1. Direct antimicrobial activity, 2. Act as a substrate for peroxidases associated with lignin polymerization and hence cell wall strengthening, 3. Via still to be determined mechanisms act as a signal for activation of expression of defense related genes, including those that result in stimulation of salicylic acid (SA) accumulation. SA is thought to act as an endogenous signal molecule that triggers expression of genes coding for several classes of pathogenesis-related proteins (PR proteins). Some of the PR proteins have antimicrobial enzymatic activities, such as glucanases and chitinases. The function of other PR proteins in defense still needs to be elucidated. Moreover, SA may potentiate the oxidative burst and thus act in a feedback loop enhancing its own synthesis. SA may also be involved in hypersensitive cell death by acting as an inhibitor of catalase, an enzyme that removes H.sub.2 O.sub.2. 4. H.sub.2 O.sub.2 may trigger production of additional defense compounds such as phytoalexins, antimicrobial low molecular weight compounds. For a review on the role of the oxidative burst and SA please see Lamb, C. and Dixon, R. A., Ann. Rev. Physiol. Plant Mol. Biol., 48: 251-275 (1997). A high level of salicylic acid is associated with disease lesion mimic symptoms. Thus, the oxidative burst is the initial signal of a pathogen's attack, but one that is not permitted to be maintained by the plant. Even plants that are able to mount a defense are usually not immune to the disease. The pathogen is often able to inflict significant damage, although the plant may not die from the disease. Plants stressed because of pathogen damage are less likely to yield well and are often more susceptible to other types of pests.
In the present invention, it is demonstrated that the transgene encoding hydrogen peroxide/reactive oxygen species producing enzyme or an oxalate degrading enzyme is able to confer a significant pathogen resistance response in sunflower, canola, and soybean. Further, pathogen resistant sunflower expressing oxalate oxidase induces the expression of pathogenesis-related genes resulting in the accumulation of high levels of PR-1, chitinase and glucanase PR proteins as well as highly elevated levels of salicylic acid. Induction of the host defense systems has been shown in numerous cases to cause broad spectrum resistance to pathogens. For example, Chen, et al. in a 1993 Science article discusses that infection of plants by a pathogen often leads to enhanced resistance to subsequent attacks by the same or even unrelated pathogens (Chen, et al., Science, 262: 1883-1886 (1993)).
A lesion mimic-like phenotype also is observed in these SMF-3 transgenic plants. Sclerotinia resistant F1 hybrids of oxalate oxidase or oxalate decarboxylase transgenic plants crossed with existing Sclerotinia tolerant sunflower lines generated near-immune plants with no lesion mimic symptoms. A similar near-immune phenomenon is also observed with F1 hybrids of canola oxalate oxidase transgenics and an existing Sclerotinia tolerant line. Thus the synergistic effect of a hydrogen peroxide/reactive oxygen species producing enzyme or an oxalate degrading enzyme in a plant with a genetic pathogen tolerance gives rise to a near immune plant. A Sclerotinia immune plant has never been described before. It is now possible to take tolerant plants and make them immune or nearly immune to Sclerotinia. A pathogen immune plant can be expected to survive and yield well under pathogen challenge without the need for externally applied control agents such as chemical fungicides. Therefore, producers are spared the expense and effort required to treat fields for disease problems. In the case of Sclerotinia, for example, current treatment protocols are only partially effective and cost prohibitive. An effective transgenic approach to Sclerotinia disease control would therefore be of significant utility.